Although the water mattress has been known for more than forty years, it has only recently gained wide acclaim and popularity. The major attractions of the water mattress are its bouyant comfort, its therapeutic value for those suffering from back ailments, and the pleasure of sleeping on a heated mattress.
Detractors of the water mattress have criticized it for its other well-known characteristics. For example, wave motion within the water that fills the mattress is only lightly damped, and may continue for some period of time. Although many people find this wave action enjoyable, others find that it is not conducive to sound sleep. Furthermore, this wave action may have a resonant frequency which is a function of the size of the mattress, the volume of water filling the mattress, and the load supported by the mattress. Many individuals have found this resonant frequency phenomena not conducive to sleep or other bedroom pursuits.
Furthermore, although the water mattress may be unexcelled in providing uniform support to the human body, this support is only available in the medial portions of the water mattress. Any weight which is applied to the outer portions of the water mattress will merely displace water towards the center of the mattress, causing the outer portion to sag or collapse. Thus, a sleeping individual may easily roll into the frame which supports the edge of the mattress. Likewise, it is difficult, if not impossible, to sit on the edge of a water mattress, even when it is supported within a rigid framework.
The prior art is replete with methods and devices for overcoming the disadvantages of the water mattress. For example, partitions have been placed within the water mattress to dampen the wave action of the water. Also, water mattresses have been filled with gel preparations which increase the viscosity of the water, and reduce the wave propagation therein.
One effective modification of the standard water mattress has been the introduction of air chambers within the water mattress. These air chambers are used to increase the support which is provided by the mattress at the extremities thereof, and also to reduce or eliminate wave propagation and persistance within the mattress. Unfortunately, an air chamber augmented water mattress is much more difficult to manufacture, due to the many extra parts which must be assembled, and the great increase in the number of welds which must be made within the mattress. The construction techniques thus become arduous and time consuming, and the cost to the consumer is quite high.
Also, the air chambers used in state of the art water mattresses are generally disposed about the perimeter of the mattress. Thus, the added damping, rigidity, and edge support is uniform throughout the mattress. This uniformity may be undesirable for a pair of sleepers sharing a mattress, is their requirements for wave damping or mattress softness differ.
Patents which exemplify the state of the prior art include: U.S. Pat. Nos. 3,766,579; 3,778,852 and 3,864,768.